1. Field of the Invention
The present invention relates to an automatic lapping apparatus for grinding or lapping a fragile piezoelectric material such as a quartz crystal wafer or the like to a predetermined thickness.
2. Description of the Prior Art
Quartz oscillators whose resonant frequencies depend on the thickness of the quartz crystals thereof are heretofore manufactured by cutting off a quartz crystal into a wafer at a certain angle with respect to a crystallographic axis thereof and lapping the quartz crystal wafer to a certain thickness for a desired resonant frequency. Various lapping apparatus for lapping quartz crystal wafers flatwise are known in the art. Of those known lapping apparatus, planetary-gear lapping apparatus are now in wide use.
One type of planetary-gear lapping apparatus includes a sun gear rotatable about its own axis and an internal gear disposed around the sun gear in spaced relationship thereto. A plurality of circular carriers, which have holes for holding quartz crystal wafers to be lapped, are thicker than the quartz crystal wafers, and have gear teeth on their outer circumferential edges, are held in mesh with the sun gear and the internal gear for planetary motion. Upper and lower lapping plates are disposed respectively on the upper and lower surfaces of the carriers. The sun gear, the internal gear, and the upper and lower lapping plates are rotated independently of each other. Between the upper and lower lapping plates, there is supplied a lapping slurry which is a mixture of water, oil, or the like and an abrasive powder of carborundum, aluminum oxide, or the like.
A process of lapping the quartz crystal wafers of piezoelectric material with such a lapping apparatus will now be described below.
The resonant frequency f0 of an AT-cut quartz oscillator which vibrates in the thickness shear mode is approximately given by: EQU f0=1670/a (KHz) (1)
where a is the thickness of the quartz crystal wafer.
Therefore, the resonant frequency of an At-cut quartz oscillator depends entirely on the thickness thereof. An AT-cut quartz crystal wafer whose resonant frequency is 16.70 MHz has a thickness of 0.1 mm. An AT-cut quartz crystal wafer having a resonant frequency of 16.71 MHz is 0.09994 mm thick. Therefore, the thickness of an At-cut quartz crystal wafer has to be reduced 0.00006 mm in order to increase the resonant frequency by 10 KHz. Stated otherwise, in order to lap a quartz crystal wafer with an accuracy in terms of frequency increments of 10 KHz, the thickness of the quartz crystal wafer has to be controlled in steps of 0.00006 mm.
It is therefore desired that lapping apparatus for quartz crystal wafers be capable of controlling the wafer thickness highly accurately and stopping the lapping plates automatically when the wafer is lapped to a desired thickness.
One relatively simple lapping control process which has been carried out on a conventional lapping apparatus effects empirical control of the lapping time under a certain load at a certain rotational speed for lapping a quartz crystal wafer to a desired thickness. However, such a method is unable to meet thickness requirements for quartz crystal wafers for use as quartz oscillators. An attempt to increase the thickness accuracy with this lapping control process requires the lapping apparatus to be stopped frequently and the quartz crystal wafers to be removed for the measurement of their resonant frequencies to check the thicknesses.
When the upper and lower lapping plates are detached for the removal of the quartz crystal wafers, many of them tend to be dislodged from the carries due to the viscosity and surface tension of the lapping slurry. After the resonant frequencies of the quartz crystal wafers have been measured, it is necessary to put all the quartz crystal wafers back on the carriers before they start being lapped again. The reattaching process is tedious and time-consuming, and results in a low efficiency. Particularly, lapping apparatus for industrial use, which simultaneously lap many wafers, ranging from several hundred wafers to one thousand and several hundred wafers, would find it impractical to have all the wafers removed for measurement in the middle of a lapping process.
It is also known to employ an air gage or an electrostatic capacitance detector for measuring the distance between the upper and lower lapping plates to indirectly measure the thickness of quartz crystal wafers therebetween, or to employ a mechanical stopper of hard material such as diamond between the upper and lower lapping plates so that the quartz crystal wafers will not be lapped down below a certain given thickness. These processes are however only capable of controlling the wafer thickness with an accuracy of about 0.005 mm at maximum, which is far lower than the accuracy required for quartz crystal wafers.
According to another lapping process, the resonant frequency of a piezoelectric material such as a quartz crystal wafer is measured while the quartz crystal wafer is being lapped, so that the quartz crystal wafer can be lapped highly accurately. More specifically an electrode is disposed on the upper lapping plate, for example, to provide an electric coupling to a quartz crystal wafer being lapped. A high-frequency signal voltage is applied to the electrode, and the frequency of the signal is varied in a given frequency range.
When the quartz crystal wafer is present immediately below the electrode and the resonant frequency of the quartz crystal wafer coincides with the frequency of the high-frequency signal applied to the electrode, the apparent impedance of the quartz crystal wafer is very low and corresponds t the equivalent resistance of the equivalent circuit of the quart crystal wafer. Therefore, monitoring the impedance of the quartz crystal wafer while it is being lapped allows the operator to know the resonant frequency of the quartz crystal wafer, and hence to determine when to finish the lapping operation.
A lapping apparatus for a quartz crystal wafer based on the above principle is disclosed in U.S. Pat. No. 4,407,094, for example. The disclosed lapping apparatus is shown in FIG. 3 of the accompanying drawings. An output signal from a sweep oscillator 126 is applied through a pin diode 120 to an electrode. The voltage applied to the pin diode 120 is also applied through a terminal 122, a 2:1 voltage divider 152, and an envelope detector 154 to one input terminal of a comparator 156. The voltage produced from the pin diode 120, i.e., the voltage applied to the electrode, is applied through a terminal 124 and an envelope detector 158 to the other input terminal of the comparator 156.
An output signal from the comparator 156 is used to control the impedance of the pin diode 120 to equalize the voltage applied to the electrode to 1/2 of the output voltage of the sweep frequency generator 126.
The output signal from the comparator 156 is compared with a preset voltage from a variable resistor 162 by a comparator 160, whose output signal is used to control a switch 130. The switch 130 is connected between the terminal of the electrode to which the voltage from the pin diode 120 is applied and a signal processor which is connected to the circuit arrangement shown in FIG. 3. When the impedance of the quartz crystal wafer below the electrode becomes smaller than a predetermined value, the switch 130 is closed to allow the signal processor to operate.
With the proposed circuit arrangement, the impedance of the pin diode 120 connected to the output terminal of the sweep oscillator 126 is controlled in order to absorb large changes in the impedance of the quartz crystal wafer below the electrode.
Since, however, the voltage across the pin diode 120 is applied to the comparator 156 and closed-loop control is effected to control the impedance of the pin diode 120 based on the output signal from the comparator 156, the time constant of the control circuit tends to be large. The upper and lower lapping plates rotate at high speed, and the quartz crystal wafer is positioned below the electrode only for a very short period of time. Therefore, if the time constant is too large, the resonance of the quartz crystal wafer cannot reliably be detected.
Varying the impedance of the pin diode 120 lessen variations in the voltage level applied to the electrode, with the advantage that noise in short periods of time can be masked and removed. This advantage however poses a problem in that when the impedance varies in a very short period of time as when the quartz crystal wafer leaves from below the electrode, such an impedance change cannot be detected.
For example, even if the impedance of the quartz crystal wafer varies as 5 V in a period of 20 ms as shown in FIG. 4, the voltage will be reduced to 3 V in a period of 5 ms as shown in FIG. 5 and applied to the signal processor of a following stage.
If a change of 5 V in a period of 2 ms as shown in FIG. 6 is applied as a change of 4 v in a period of 1.5 ms as shown in FIG. 7 to the signal processor. It is difficult to distinguish between such an impedance change and an abrupt dip due to resonance of the quartz crystal wafer.